In the movie The Fighter,
Micky is seen drilling into a bucket of rice with his fist. I have seen speeding ball exercise, rope jumping, running, but this is the first time and only time I have seen a boxer training like this.

The rice bucket exercise is a commonly used practice with athletes trying to develop strong wrists. If you're wondering how to strengthen wrists for boxing, this unconventional approach can make a huge difference.

As a side note a lot of rock climbers do this as well. It helps with general hand and wrist strength, especially in a way that is antagonistic to the exercises an athlete may typically do (helps prevent long term injury)
– wearebobMay 20 at 18:37

Steve Carlton, a famous Major League pitcher from the late 60s to the mid 80s, was known for this as well.
– chepnerMay 21 at 0:25

It also (or so I've heard) builds calluses on the knuckles, so that punching something doesn't hurt your fist as much
– BlueRaja - Danny PflughoeftMay 21 at 22:18

1

@wearebob rock climbers should also be flexing/flicking the fingers open as hard as they flexed them closed while grabbing the rice after grabbing. The small difference focuses more on increasing finger extensor strength for holding onto small/poorly-angled surfaces (e.g. pinching or crimping) rather than just toughening up the skin and bones of the hand (more useful for closed-fist striking).
– TylerHMay 22 at 18:07

Sort of, Kung Fu and various martial arts practitioners punch sand which has no give.
"Iron Palm" or conditioning the hands, canvas bags filled with gravel to be struck over and over again. Punching rice is no different than punching a punching bag. Conditioning your knuckles for greater hardness. Others do it or gaining grip strength by grinding it in their hands. It's called "Rice Digging"